This invention relates to an output control system for internal combustion engines for use in automotive vehicles and more particularly to an output control system of this kind which is intended to properly control the output torque of the engine in response to the operation of a variable capacity compressor driven by the engine.
Conventionally, output control systems for internal combustion engines for use in automotive vehicles are known, e.g. by Japanese Provisional Patent Publication (Kokai) No. 57-56645 and Japanese Provisional Patent Publication (Kokai) No. 62-41951 in which the amount of intake air is controlled in a feedback manner to maintain the idling engine rotational speed at a desired value when the engine is at idle, while at the same time the amount of intake air is varied depending upon the operation of a compressor for air-conditioning so that the desired idling engine rotational speed may be maintained even during operation of the compressor, which acts as load on the engine. Specifically, according to Japanese Kokai No. 57-566445, the intake air amount is increased or decreased by a predetermined amount in response to turning-on or -off of an electromagnetic switch for connecting or disconnecting the compressor to or from the engine. On the other hand, according to Japanese Kokai No. 62-41951, the intake air amount is varied in response to the discharge pressure of the compressor.
However, the above conventional control systems have the inconvenience that when they are applied to a compressor of variable capacity type, the intake air amount cannot be determined in response to operating conditions of the compressor, and therefore the idling engine rotational speed cannot be properly controlled, which results in increased fuel consumption, engine stalling, etc.
FIG. 1 shows an example of characteristic of load acted on the engine by the compressor for driving same (hereinafter simply referred to as "load torque"), of which the capacity is variable depending on the stroke of its pistons. In the figure, the numerals 4-15 represent values of the discharge pressure Pd, and the broken lines enclosing the numerals indicate the distribution ranges of the discharge pressure Pd values plotted with respect to the stroke F.sub.STR. Insofar as the engine rotational speed is constant, the stroke F.sub.STR varies depending upon the relationship between suction pressure of refrigerant drawn into the compressor and discharge pressure Pd of refrigerant being discharged from the compressor. Therefore, the load torque raries in response to the stroke F.sub.STR and the discharge pressure Pd, as shown in the figure. According to the figure, the stroke F.sub.STR varies with the discherge pressure up to 70% to 80% of the maximum capacity of the compressor, and accordingly the load torque is increased in proportion to the stroke F.sub.STR. In the range where the load torque is even higher, it varies depending on both the stroke F.sub.STR and the discharge pressure Pd. Therefore, even if the engine output is increased in response to operation of the compressor by a predetermined degree or in accordance with the discharge pressure alone as in the above conventional control systems, it is impossible to fully respond to variation in the load torque and hence set the amount of intake air to a proper value.
In the meanwhile, the load torque not only varies with the stroke of pistons and the discharge pressure but also varies almost in proportion to the engine rotational speed. Therefore, even if the operation of the compressor undergoes change under the same operating conditions, the amount of variation of the load torque resulting from the change is different if the engine rotational speed is different. That is, the higher the engine rotational speed, the greater the amount of variation of the load torque.
Therefore, when the engine is operating in a region where the engine rotational speed is higher than the idling engine rotational speed while the compressor is in operation, if the incremental value of intake air amount is set based upon the piston stroke and the discharge pressure alone, the intake air amount becomes insufficient, which in turn results in insufficient output torque of the engine. As a result, torque shock may occur when the operation of the engine is started or the capacity thereof is increased. Further, the engine rotational speed may abnormally fluctuate; for instance, when the vehicle is cruising, the engine rotational speed may drop upon start of the compressor to make it difficult for the vehicle to run at a constant speed.